1. Field of the Invention
This invention relates to an improved infrared seeker assembly, and more particularly to one having an improved focal plane platform construction.
2. Discussion
Infrared detection systems are often used in conjunction with systems for sensing electromagnetic radiation in the wavelength range of one to fifteen micrometers. Because many such detection systems have photoconductive detector arrays which are most sensitive when operated at about 80.degree. K., a cooling system is required to produce and maintain the required low operating temperatures. Typically, such cooling systems either take the form of a cryostat utilizing the Joule-Thompson effect, or a Stirling cycle cryoengine. The cooling systems are used in conjunction with an evacuated dewar in which the electromagnetic detector is placed. The dewar is evacuated to remove thermally conductive gases which would otherwise occupy the volume surrounding the detector so that potential heat load through convection and conduction is minimized. The evacuated dewar also prevents moisture from condensing on the detector. The detector is cooled by placing an indented region ("coldwell") of the dewar in contact with an expansion chamber ("expander") of the cryogenic cooling system. Commonly, the expander has a cylindrical tube ("coldfinger") having an end which is cooled and which supports a focal plane platform upon which the detector and related components, such as integrated circuit amplifier and readout chips, are mounted. Alternately, the dewar can be constructed without a coldfinger such that the detector is mechanically supported directly by the focal plane platform. The cooling system produces cyclical cooling by sequential compression of a working fluid such as helium, removal of the heat generated during compression of the working fluid, and subsequent expansion of the working fluid. Thermal energy is withdrawn from the detector through the focal plane platform which is in thermally conductive communication with the cooling system. Since the cooling system is in thermal communication with the focal plane platform, expansion of the working fluid within the coldwell causes thermal energy to be withdrawn from the detector.
In order to produce efficient conductive withdrawal of thermal energy from an electromagnetic detector, the focal plane platform on which the detector is mounted must be fabricated from a material, or composition of materials, possessing specific metallurgical properties. Ideally, these properties include high strength, a high modulus of elasticity and high thermal conductivity. Additionally, the focal plane platform must produce low thermal distortion characteristics to minimize premature detector failures.
A number of design constraints affect the design of the focal plane platform. Since the focal plane platform is a structural support member, it must have sufficient bending stiffness to minimize mechanical deflection of the electromagnetic detector and the amplifier chip. Such requirements become particularly significant when the infrared seeker assembly is subjected to intense vibration and high levels of boost-phase acceleration. Another significant design parameter is the extent to which heat is transferred through the focal plane platform.
As previously mentioned, typical photoconductive detector-arrays are most sensitive at an operating temperature of about 80.degree. K. However, the bipolar silicon amplifier circuits, provided for electrically interfacing the detectors to remote multiplexer circuits do not operate effectively at temperatures below about 150.degree.-200.degree. K. Cooling the silicon amplifier circuit to the same temperature as the detector causes "freeze-out" of the dopants in the silicon, thus reducing transistor gain levels. Such "freeze-out" causes the transistors to become practically inoperative for electrically coupling the detector to the multiplexer circuit.
In recent years, focal plane platforms have been fabricated from various materials. Titanium, tungsten, copper, KOVAR and beryllium have been used, but unfortunately the platforms have not been designed to thermally isolate the readout circuit from the detector under thermal cycling conditions. While such materials provide an adequate thermal heat sink for the detector, none provide the thermal isolation necessary to eliminate the freeze-out problem which can render conventional silicon readout circuits insensitive and/or inoperative.